Medical diagnostic imaging systems in general acquire signals from the interior of a patient's body. To generate the image, the location of the source and the relative intensity of each of the signals must be determined. The intensities of the signals are measured and converted into gray scales.
For example, ultrasonic scanning systems acquire data to provide medical images of the interior of patients. In general, the systems use transducers to transmit ultrasonic waves in the order of several Mhz. in frequency into a subject or a patient. "Echo" signals are received responsive to these transmitted ultrasonic waves and are used as data for the images. The transducers are positioned juxtaposed to the patient's body. The intensities of the received echo signals are measured and converted into gray scale determinations ranging from white to black. The location of source, that is the echo generating material (i.e., boundaries of organs and the like) is basically determined by the time required for the echo to return after the original signal is transmitted. The acquisition of the intensity data correlated to location in the body of the source of the signal enables obtaining intensity values for image pixels which correspond to body locations as is well known when providing images.
As the transmitted waves travel through the body they are attenuated. The received echo signals are relatively weak and require amplification. In practice, the gain of the amplification is varied by time gain compensation (TGC) circuitry which overcomes the attenuation of the echo signal caused by the distance the origin of the echo signal is from the transducer.
The intensities of the echo signals in medical imaging systems are characterized by dynamic range; i.e., a gray range greatly exceeding display capabilities. The echo intensities depend upon such things as:
1. intensity of the target or source; i.e., the transmitted signal at the point from which the echo signal emanates;
2. the target impedance mismatch to surroundings;
3. the target's geometrical orientation; and
4. attenuation of the acoustic signal by the tissue.
Diagnostic pulse echo systems using ultrasound for examination of targets deep within the body will typically produce echo signals spanning a dynamic intensity amplitude range of 100 dB or more. In any given range segment, target acoustic impedance differences from the surroundings and geometrical orientation will provide variations in echo strength of 30-50 dB. This represents the desired target information. The additional 50-70 dB variations originating from the tissue attenuation over the total path length represents an unwanted component. These additional 50-70 dB variations are taken care of by the TGC circuitry which compensates for intensity variations of the echo due to absorption. Direct observation of signals in the 30-50 dB dynamic range is not practical with conventional display devices. Therefore, it is apparent that it is not practical to enable viewing more than a small segment of the entire dynamic range at any one time.
Thus, ultrasound systems generate images by converting echoes of different amplitudes into image points of different brightness. The brightness is expressed by a graduated gray scale where lower amplitude echoes are resolved as darker shades of gray and higher amplitude echoes as brighter shades. The assignment of a given gray shade to a particular echo amplitude is arbitrary and it is determined by an echo to gray shade conversion curve employed during processing of the data. In fact, there are over 1,000 shades of gray in ultrasound images after the TGC. The present state of the art ultrasound systems reduce this 1,000 shades of gray by methods such as logrithmically compressing the data; i.e., a variable gain is used as a function of the signal level. The higher the signal level, the lower the gain. Thus, for example, the differential gains at high input levels may be only about 0.01 of the gain at the lower signal input levels.
Therefore, when logrithmic data compression systems are used, clinical information that may be expressed as small local variations of high signal input levels are lost.
Another prior art solution to the problem of adapting the 1,000 shades of gray to what is presently available with regard to TV monitors and the limits of human vision; i.e., approximately 100 shades of gray, is off-line windowing. In using such windowing, the operator manually selects the optimal signal input level range to be displayed as the full range of the monitor. This method is faulty for ultrasound because:
1. the optimum window is local in nature thereby optimizing the image at a certain region will generally result in causing a deterioration of the image at other regions; and
2. the method is difficult to implement on line.